As with most of Dickens' work, Oliver Twist is used to bring the public's attention to various contemporary social evils, including the workhouse, child labour and the recruitment of children as criminals.
Adaptations of the novel tend to simplify the original story, which is as follows:
Warning: wikipedia contains spoilers
Oliver is a boy born in a workhouse, who has no idea of his parents' identity. By pure chance he is chosen as a scapegoat by the other starving boys, and is made to go and ask for an extra helping at a mealtime ("Please, sir, I want some more."). As a result of this breach of etiquette, he is "sold" by the workhouse as an undertaker's apprentice. The cruelty he suffers at the hands of an older apprentice causes him to run away, and he finds his way to London, where he is taken under the wing of The Artful Dodger, a boy criminal. The Dodger introduces Oliver into his circle of friends, who include Fagin, the Jew, a criminal mastermind, and his brutal ally, Bill Sykes. Oliver is taught crimes such as picking pockets and forced to take part in them. He is shown kindness by Bill's girlfriend, Nancy.
After a robbery that goes wrong, Oliver is taken into the home of a wealthy man, Mr Brownlow. Unknown to them, efforts are being made by Oliver's half-brother, Monks, to locate him and prevent him from obtaining his inheritance, but Mr Brownlow soon begins to suspect that Oliver is the son of his niece. Sykes and Nancy snatch Oliver back, and Sykes takes him on a burglary, planning to get him a criminal record as a favour to Monks. But Oliver is left behind and restored to Mr Brownlow. Sykes murders Nancy and is himself killed while being pursued by the police. Fagin is arrested and hanged[?] for his crimes.
The way the book is normally interpreted on screen causes modern readers to focus on Bill Sykes as the villain. They thus fail to recognise how Fagin has trained Sykes and made him what he is; part of Dickens' message is that he might have done the same with Oliver had chance not intervened.